This project will examine the interaction between the Leishmania parasite and its target host cell, the macrophage, which are relevant to disease. Leishmania genes that have a potential role in disease will be identified by examination of both Leishmania and macrophage gene expression responses to parasite infection of the macrophage. 1) Mouse macrophage gene expression responses to Leishmania infection in vitro will be determined using DNA micro arrays for genes that are expressed by activated macrophages. The responses of macrophages from mice that are susceptible or resistant will be compared. The macrophage responses to Leishmania strains that cause cutaneous or visceral, and/or virulent or avirulent disease will also be compared. These studies will identify macrophage gene expression responses to Leishmania infection, which correlate with susceptibility, or resistance to Leishmania and disease severity. They will generate macrophage gene expression profiles that will be used to identify and assess the role of Leishmania genes in macrophage infection and leishmanial disease. 2) Leishmania gene expression responses that are associated with macrophage infection will be identified using DNA micro assays and 2D-PAGE. These studies will identify genes with expression responses that correlate with life-cycle stage disease (virulent, avirulent, cutaneous, or visceral) and macrophage responses. These data will be used to select Leishmania gene with a potential role in infection and disease for functional analyses. 3) Recombinant Leishmania with altered expression of selected genes will be examined for the effects on parasite gene expression at the RNA and protein level and their ability to Infect and grow in macrophages. 4) The cytokine responses will be examined to assess the Th1 vs. Th2 responses that correlate with control of or susceptibility to Leishmania.